Photovoltaic modules are used in solar energy systems for converting sunlight to electricity. Manufacturers and testing centers routinely test PV module products to determine their performance characteristics and power output.
Laboratory-based indoor testing of PV modules requires a solar simulator apparatus that can expose a module to simulated sunlight. Many tests are performed with a pulsed or “flash” tester, which exposes a test module to a brief flash of light and measures the module's current versus voltage relationship (“I-V curve”) during the short duration of the flash. However, other tests require steady-state simulators, which provide continuous light exposure over longer durations. Various tests requiring steady-state solar simulators are described, for example, in International Electrotechnical Commission (IEC) standards 61215 and 61646, which are widely adopted in the PV industry.
It is known that the performance characteristics of PV modules may change upon continuous exposure to sunlight, particularly for “thin-film” PV technologies such as those based on amorphous silicon, CIGS, CdTe, or organic materials. Light-induced variations include both short-term effects, which may either improve or degrade performance, and long-term effects, which generally cause slow degradation. Therefore, in order to accurately assess PV module performance, manufacturers and testing centers may subject test modules to continuous light exposure for sustained periods. This is referred to as “light soaking” Light soaking durations vary widely depending on application, but can be up to hundreds of hours for module stabilization tests such as described in IEC 61646.
An apparatus used for light soaking is, generally, a form of a steady-state solar simulator. Typically, however, the specifications of an apparatus used for light soaking purposes will differ from those of a steady-state solar simulator used, for example, for I-V testing, due to the differences in the intended applications.
Various types of equipment are in use for light soaking. The simplest equipment uses bright lamps arranged over a horizontal test bed where the module is placed. In some cases lamps are arranged in a vertical plane and illuminate a vertically oriented test module. Such free-standing lamp systems are typically operated in a dedicated room or enclosed area, due to the need for eye protection when working near the lamps. Recently, light soaking chambers have been developed which completely enclose the lamps and test modules.
Existing light soaking systems have a number of shortcomings, particularly for applications where many modules must be tested. Free-standing arrays of lamps, as already noted, require dedicated rooms or enclosed areas, and therefore use significant floor space per tested module—particularly for horizontally oriented test beds. Furthermore, such systems are generally energy inefficient, since the lamps typically illuminate an area much larger than the test module in order to achieve uniform illumination over the area of the module. While chamber-based systems have several advantages versus free-standing arrays of lamps, existing light soaking chambers are based on horizontal test bed designs, and therefore occupy significant floor space per tested module. Furthermore, controlling the temperature and the temperature uniformity of the tested modules is difficult with existing systems, particularly if the module temperature is to be adjusted over a wide range for different test purposes.
In view of the above shortcomings of existing light soaking systems, there is a need for an improved light soaking apparatus for PV modules.